1. Field of the Invention
The invention relates to an engine control device that makes a malfunction diagnosis of an exhaust gas recirculation system during fuel cutoff, and also relates to an engine control method.
2. Description of Related Art
An exhaust gas recirculation (EGR) system that recirculates a part of exhaust gas into intake air is known as a system installed on a vehicle, or the like. The EGR system has an EGR passage that connects an exhaust passage and an intake passage of the engine, and an EGR valve mounted in the EGR passage. The EGR system is configured to increase and reduce the EGR amount, i.e., the amount of gas recirculated from the exhaust passage into the intake passage, through control of the degree of opening of the EGR valve.
In a system as described in Japanese Patent Application Publication No. 2009-257280 (JP 2009-257280 A), a malfunction diagnosis of the EGR system as described above is conducted during fuel cutoff. The malfunction diagnosis is performed by forcedly opening and closing the EGR valve during fuel cutoff, and determining whether the EGR system is functioning normally, depending on whether a change commensurate with an increase or reduction of the EGR amount due to the opening or closing of the EGR valve appears in a detection value of an air flow meter or an intake pressure sensor installed in the intake passage.
In this connection, even if the fuel cutoff is started, combustion gas in the interior of the exhaust passage is not immediately replaced by new air, but remains for a while. If the malfunction diagnosis of the EGR system is conducted in this condition, the combustion gas remaining in the exhaust gas is introduced into the intake air, in accordance with opening and closing of the EGR valve. If the engine returns from the fuel cutoff state at this time, combustion is resumed in a condition where air containing combustion gas is introduced in the cylinder, which may result in unstable combustion.
When the engine returns from the fuel cutoff state, combustion is suddenly resumed and engine torque is generated, from a condition where combustion has not been done, and the engine torque is equal to zero. Therefore, a step-like change appears in torque at the time of return from fuel cutoff, and the driver recognizes the step-like change as torque shock. As a measure to avoid or alleviate the torque shock, control for retarding the ignition timing at the time of the initial explosion after the return, namely, ignition timing retard control at the time of return from fuel cutoff, is performed. In the following, the retard amount of the ignition timing at the initial explosion after the return is denoted as “initial retard amount”.
FIG. 7 shows one example of control mode of the ignition timing retard control as described above. In a time chart of FIG. 7, time t1 denotes the time of the initial explosion after return from fuel cutoff, and time t2 indicates the time of the second explosion after the return. If the ignition timing is not retarded at the initial explosion after the return, as indicated by a broken line in FIG. 7, the engine torque rapidly rises after the initial explosion, and torque shock occurs. On the other hand, if the ignition timing is retarded at the initial explosion after the return, as indicated by a solid line in FIG. 7, the rise of the engine torque after the initial explosion is reduced by an amount of “ΔTq”, and the step-like change in torque at the time of return from fuel cutoff is reduced.
On the other hand, when the engine returns from the fuel cutoff state immediately after the malfunction diagnosis of the EGR system as described above, combustion is resumed in a condition where the amount of new air flowing into the cylinder is larger than usual. In this case, even if the ignition timing retard control is performed as usual at the time of return from fuel cutoff, torque shock may not be sufficiently alleviated.
FIG. 8 shows one example of control mode of return-time ignition timing retard control performed when the engine returns from a fuel cutoff state immediately after a malfunction diagnosis. In the engine returning from the fuel cutoff state at this time, the combustion state is improved with an increase in the amount of new air flowing into the cylinder. As a result, the sensitivity of engine torque to the ignition timing is enhanced. Namely, if the ignition timing at the initial explosion after the return is retarded by the same amount as usual, the amount of reduction ΔTq of engine torque becomes larger than usual. In this case, since the rise of the engine torque at the initial explosion after the return is suppressed by a larger degree, the engine torque rises rapidly after the ignition timing stops being retarded (in the example of FIG. 8, after the second explosion after the return), and the driver may recognize the rapid rise as torque shock.